Anatolian tectonic plate, north anatolian fault, historical earthquakes Greece, Attica, Earthquakes, Tsunami, , Hurricanes, Volcanic Eruptions and other Natural and Man-Made Hazards and Disasters - by Dr. George Pararas Carayannis

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The Earthquake of 12 November 1999 in Turkey

George Pararas-Carayannis

Introduction

On November 12, 1999, another devastating earthquake with magnitude 7.2 struck a hilly region of northwestern Turkey spreading death and destruction to Duze, a city of 80,000, the city of Bolu and surrounding villages. This was the second strongest earthquake to strike Northern Turkey in less than three months, since the powerful 7.8 magnitude quake of August 17, 1999 struck the densely populated area near Izmit and killed more than 17,000 people. The November 12, 1999 earthquake was not an aftershock but a different seismic event, apparently along the same western segment of the North Anatolian Fault system. It was felt widely throughout the region, in Istanbul, Ankara and the Eastern Mediterranean coastline.

Earthquake Epicenter, Origin Time, Magnitude and Aftershocks

The epicenter of the November 12, 1999 earthquake was at 40.8 N , 31.6 E (Strasburg Geodynamic Institute) in the Duze region, near the city of Bolu , which is about 170 Km southeast of Istanbul and approximately 45 miles east of the region hit by the August earthquake. Its magnitude was estimated at 7.2 on the Richter scale. The Strasburg Geodynamic Institute gave the magnitude as 7.3.

Within a two hour period following the main quake, 69 aftershocks were recorded, five of which had magnitudes greater than 5. Of these, 30 aftershocks had magnitudes ranging from 3-3.9, 33 had magnitudes ranging from 4-4.9, and 6 ranging from 4.9-5.4 (Kandilli Observatory and Research Institute). Aftershocks continued for weeks and months.

Geological Instability of the regio

 

Modified After Nafi Toksoz of MIT/ERL

The excessive seismicity of this particular region has been described previously in the report of the August 17, 1999 earthquake. Briefly restated, the North Anatolian fault is a major fracture that transverses the Northern part of Asia Minor and marks the boundary between the Anatolian tectonic plate and the larger Eurasian continental block. Because of this unstable tectonic system, this area is considered as one of the most seismically active zones of the world.

Tectonic and Geologic Setting

The North Anatolian Fault Zone is the most prominent active fault system in Turkey along which numerous large earthquakes have occurred throughout history. It extends for 1200 km in an approximate east west direction. It has resulted from movements of the Eurasian tectonic plate grinding against the Anatolian tectonic plate. At least since 1930, when instrumental measurements begun in the area, the Turkish portion of the Anatolian plate is pushing at the rate of about 3 cm/year against the Greek portion of the plate in the vicinity of the Northern Aegean sea near the island of Samothrace. In fact, earthquakes in the Northern Aegean followed the August 17, Turkish earthquake, releasing the strain that had accumulated. The net tectonic movements suggest also a rotation of the Turkish portion of the tectonic plate in a counterclockwise direction.

As with the August 17 event, the quake of November 12, 1999 occurred along the long, east-west trending, great North Anatolian fault zone (NAFZ) in Northwestern Turkey. As we reported, NAFZ has many similarities to the San Andreas fault system in California. Like the San Andreas, earthquakes along the Northern Anatolian Fault involve primarily horizontal ground motions.

 

Recent Historical Earthquake Activity Along the Northern Anatolian Fault
Source: Kandilli Observatory and Research Institute (modified)

The earthquake of November 12, 1999 occurred on the northern branch of NAFZ and broke a 60-80 km segment between Duze and Bolu. This happened also along a section of a previously identified seismic gap where an earthquake had been expected, particularly because of the additional strain that the August 17 event had added. Following the August 17 quake, there had been speculation as to whether sufficient stress had been added to either east or west segments at either end of its rupture along the North Anatolian fault and which of these adjacent segments would rupture next. The November 12, 1999 quake demonstrated the rupture and horizontal movement, this time, was toward the east.

Progressive failure on the North Anatolian fault since 1939 (Stein et al. 1996)

Earthquake's Surface Rupture and Ground Displacements

The duration of ground motions from the November quake were a great deal shorter than those of the August quake and lasted somewhere between 15 to 20 seconds. Although field surveys have not been completed, ground displacements are estimated to cover an area of about 60-80 km in length along the fault. The extent of horizontal ground displacements are not known yet, nor whether they were any vertical movements. A single vertical displacement of about 1.5 m was reported in the Bolu area but his may have been an anomaly, perhaps the result of liquefaction or some surface effect, as most of the movements along the NAFZ are primarily horizontal. However, what is known with certainty is that the direction of seismic activity had moved in an eastward direction, for this event.

Did the November 12, 1999 Earthquake occur on the NAFZ or on a parallel fault?

The NAFZ is a very large and well known system but there has been speculation that perhaps this latest November 12, 1999 quake occurred on a parallel branch to the major fault. Field surveys and studies of aftershock epicenter and hypocenter distributions will probably shed light on whether this latest earthquake occurred on a branch of the NAFZ or not. However, it would seem that the August earthquake would have relaxed the strain on parallel to the NAFZ faults. Such relaxation of parallel faults occurred along the northern portion of the San Andreas fault system following the Great San Francisco 1906 earthquake, while the strain was increased on end-to-end faults. Therefore, based on this past observation of seismic activity release in a different area of the world, we may conclude tentatively that the latest November earthquake in Turkey must have occurred on an end-to-end adjacent segment of the same NAFZ fault, which had been strained by the August quake.

In the report of the August 17, 1999 earthquake we provided the historical background of seismic activity in the 20th century for western Turkey along the Northern Anatolian Fault. We indicated that, beginning in 1939, there have been numerous large earthquakes with Richter magnitudes of over 6.7. These have struck in progression along adjacent segments of the the NAFZ fault. The August 17, 1999 earthquake was the eleventh in a series of such events occurring in progression. The most recent November 12, 1999 quake was the twelfth.

Did the November 12, 1999 Earthquake Occur Along a Known Seismic Gap?

Starting with the 1939 event, twelve earthquakes have now broken segments of the Northern Anatolian Fault fault in both eastward and westward direction. Our review of historical seismic data shows that between 1939 and 1944 there was an active westward trend in the seismic activity with a resulting surface rupture of 600 km of adjacent fault. Subsequently, the westward trend of earthquakes slowed down. Earthquakes occurring in 1957 and 1967 ruptured an additional adjacent 100 km of fault but there was separated activity further west during 1963 and 1964. There was a long seismic gap separating the 1967 quake and the 1963 and 1964 quakes. The August 17, 1999 quake predictably occurred along a portion of this segment. Similarly, the November 12, 1999 quake occurred along the previously unbroken segment of this gap.

Was All the Strain on the Seismic Gap Released by the November 12, 1999 Earthquake?

The westernmost end of the segment that ruptured by the November quake appears to end near the eastern most end of the segment that ruptured by the quake in August. This would indicate that the gap for this particular region has closed and that most of the seismic strain was released by the November 12, 1999 quake . However, it is not known with certainty at this time whether the closure of the gap has been complete. There is a possibility that still a portion to the west of the present rupture, in the area between Sapantza and Akgiazi, a small gap may still exist and that strain may have accumulated. Another seismic gap also remains under the Sea of Marmara, south of Istanbul, where an earthquake and even a small tsunami may occur in the future.

It is quite possible that all the seismic strain was not released by the November event and that some additional future seismic event will release the remaining strain. A moderate size earthquake would be expected to release the remainder of the stress in the area, but such an earthquake may not necessarily happen soon.

Damage from the earthquake

Death Toll and Extent of Damage

As of November 18, 1999, the death toll was 619 but expected to continue to rise because this is a densely populated area. More than 3,000 people were reported injured and several hundreds more were missing. The November 12, 1999 earthquake collapsed buildings and mosques and tore apart highways. Unconfirmed reports estimate damage at around $10 billion. There was extensive damage throughout the area. Many buildings collapsed at the city of Bolu , near the earthquake's epicenter, and in Smyrna some distance away. There was extensive damage at the village of Kayanasli.,15 km from Duze, on the way to Bolu.

Will there be earthquakes elsewhere in Turkey?

Turkey is located on a very seismic area of the globe where earthquakes occur with certain frequency. The present trend of seismicity indicates future activity to move in a western direction. We indicated that small seismic gaps exist in the area west from where the November earthquake ruptured the surface. There are also some secondary fault systems where moderate earthquakes are possible in the future. One small seismic gap, approximately 150 km long, exists presently under the Sea of Marmara about 25 km south of Istanbul. A moderate to large earthquake could occur along this gap in the future and a tsunami could also be generated. There is no way to predict at the present time when this may exactly happen. Scientists believe that such an earthquake could occur at any time in the next 50 years.

Conclusions

The November 12, 199 earthquake was a different major earthquake and not an aftershock of the August 17, 1999 earthquake to the east. In all probability it occurred along an end-to-end segment of the same fault system rather than on a parallel collateral fault. Aftershocks are expected to continue in the immediate area and some may be larger in magnitude. There is no reason for an earthquake to occur in Greece as a result of this November earthquake and the September earthquake in Central Greece was unrelated. However, the northern Aegean could experience some additional earthquake activity, as in the recent past. A number of grabbens, fault offsets and other structural topomorhological features at the bottom of the Sea of Marmara indicate that seismic activity and movements of branches of the North Anatolian fault extend under the sea. A seismic gap has been identified in the area south of Istanbul where an earthquake can be expected in the next 50 years. However, this does not mean that it cannot happen sooner or at any time.

References

Ross S. Stein, Aykut A. Barka and James H Dieterich, Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering, Geophysical Journal International, Vol. 128, 594-604, 1996.


M. Nafi Toksoz, A.F. Shakal, and Andrew J. Michael,
Space-time migration of earthquakes along the North Anatolian fault zone and seismic gap, Pageoph, Vol. 117, 1258-1270, 1979.


Earthquake Information from National Earthquake Information Center (NEIC) (USGS)


Earthquake Information from Bogazici University, Kandilli Observatory and Research Institute.


Special page on the earthquake by IRIS (Incorporated Research Institutions for Seismology)

U. Kuran and A.C.Yalciner (1993),
Crack Propagations, Earthquakes and Tsunamis in the Vicinity of Anatolia, in Tsunamis in the World, ed.by S.Tinti, pp.159-175

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